[Mesa-dev] [Bug 96518] [llvmpipe] piglit linestipple regression

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=96518

--- Comment #1 from Roland Scheidegger <sroland at vmware.com> ---
I suppose I missed that when I did a piglit run...
This is fairly interesting, it seems to be due to inaccurate float math.
The stipple code gets for position x0,x1 10.5, 34.5 leading to a length 24.
However, with the MADs used for vertex transform replaced by fmas, I get
position 10.500002, 34.500000 instead. Hence length 23.999998. And subsequently
patterns getting different (I think it makes no difference on the first
segment, but the second due to the stipple->counter += length line).

Not sure though why the results would be seemingly less accurate with fma. (And
not sure there shouldn't be some different behavior wrt rounding in the line
stipple code?)

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